The DTT Toroidal Field (TF) coil winding packs are designed to be wound in double-pancakes. The DTT TF-Joints, designed by ASG Genova in a “praying hands” configuration, will provide the “pancake-to-pancake” electrical connection (i.e. between the series of the superconducting Nb3Sn double-pancakes) and, in addition, the “coil-to-bus bar” electrical connection (i.e. between the TF coil terminations and bus bars conductors, bringing current from the power supply).

To limit the thermal load, the joint resistance must satisfy the maximum project-allowed requirement of 2 nΩ in self-field (i.e. zero background field). Testing is mandatory to verify that the design meets the requirement.

The two weeks test campaign of the DTT TF-Joint sample (see Fig. 1) was successfully held at the SULTAN facility at the Swiss Plasma Center. The extensive characterization included pressure-drop of the coolant (Helium), AC losses and DC measurements, and, in addition, according to the TF conductor’s characterization, comprised also 3000 electro-magnetic loading cycles and two thermal cycles to test the performance stability of the joint.

The joint resistance value has been measured, at the cryogenic temperature (4.5 K), up to the nominal operating current (42.5 kA) and at background magnetic fields up to 5 T. Its value at zero background field of 1.7 nW is below the design requirement with a good stability throughout electromagnetic and thermal cycles.

The results obtained confirmed the quality of the design and its compliance to the technical requirements.

Fig. 1 TF-Joint sample on a bench in SULTAN Lab